It has been known for some time that a hot electron transistor could potentially be operated at frequencies in excess of those achievable with conventional (diffusive) transistors. See, for instance, T. E. Bell, IEEE Spectrum, February 1986, pp. 36-38, incorporated herein by reference. Various types of hot electron transistors (HET) have been proposed. This application is concerned with one particular class of such transistors, namely, hot electron heterojunction bipolar transistors (HBTs). For a brief review, see L. F. Eastman, ibid, pp. 42-45, also incorporated herein by reference.
The flow of electrons from emitter to collector in a bipolar transistor is controlled by varying the emitter/base barrier potential by means of an applied voltage V.sub.eb, and is also a function of an externally applied voltage V.sub.bc between base and collector. Under normal operating conditions, V.sub.bc reverse biases the base/collector junction. Electrons injected from the emitter into the base of a bipolar HET have energy substantially greater than the thermal energy of the ambient electrons in the base. These "hot" electrons ideally should traverse the base and collector depletion region without undergoing significant scattering, and enter the sea of conduction electrons in the collector contact region.
As will be readily understood by those skilled in the art, substantial difficulties have to be overcome before a device of this type can function as a practical HET. Among these is the difficulty of achieving substantial hot electron transport through the base and the depletion region of the collector.
Three recently filed U.S. patent applications Ser. No. 871,494, filed Jun. 6, 1986 by J. R. Hayes et al; Ser. No. 074,127, filed Jul. 17, 1987, by A. F. J. Levi; and Ser. No. 241,279, filed Sep. 7, 1988 by A. F. J. Levi, all incorporated herein by reference), disclose means for achieving improved HETs. However, in view of the general desirability of improved characteristics such as larger DC current gain .beta. (defined as collector current I.sub.c divided by base current I.sub.b) and high cut-off frequency f.sub.T, means for achieving further improvements in HET characteristics would be of considerable significance.
In prior art HBTs the emitter stripe width is relatively large, typically at least about 1 .mu.m and, to the best of our knowledge, has not been less than 0.6 .mu.m even in a laboratory device that had quite low .beta. (from the data reported it can be deduced to have been about 16; see N. Hayama et al, IEEE Electron Device Letters, Vol. EDL-8(5), May 1987, pp. 246-248). To the best of our knowledge, the prior art has not succeeded in making a HBT which combines at room temperature a large f.sub.T (.gtoreq.80 GHz) with a large (.gtoreq.25) .beta. and small (.gtoreq.1 .mu.m) emitter stripe width. This is to be contrasted with Si bipolar transistors in which high performance and stripe widths as small as 0.35 .mu.m have been achieved (see S. Honaka et al, IEEE Transactions on Electronic Devices, Vol. ED-33, pp. 526-531, 1986). As is well known to those skilled in the art, narrow emitter stripe width is advantageous since it can, inter alia, result in lower power consumption at a constant current density (and thus make possible larger scale integration), or in increased speed at a constant current (and thus constant power).
In a prior art (diffusive) bipolar transistor electrons are injected from the emitter stripe into the base, with the base typically being substantially wider than the emitter stripe (a bipolar transistor structure is schematically depicted in FIG. 1). The base region that underlies the emitter stripe is frequently referred to as the "intrinsic" base region, and the base regions that do not underlie the emitter stripe as the "extrinsic" base regions. It is to be noted that in this context "intrinsic" has nothing to do with the dopant concentration in the base region.
Electrons injected into the intrinsic base region of a (diffusive) bipolar transistor undergo diffusive motion, and some of these minority carriers undergo recombination, a process which, inter alia, results in decreased .beta.. Recombination can take place both in the bulk base region and on the exposed surfaces of the (extrinsic) base region. Associated with any semiconductor material is an intrinsic surface recombination velocity S.sub.o. For instance, in silicon S.sub.o is approximately 10 cm/sec, whereas in GaAs it is about 10.sup.6 cm/sec.
As will be readily appreciated by those skilled in the art, surface recombination in general becomes of increasing importance as the emitter stripe width is decreased. Due to the relatively small S.sub.o of Si it is possible to make transistors that have small stripe width and yet have acceptable values of .beta.. However, due at least in part to the much larger S.sub.o of GaAs it has not been possible to similarly reduce the stripe width of GaAs HBTs without causing unacceptable degradation of .beta.. As is well known, most of the work on HBTs has so far been done on GaAs-based devices. From the above discussion it can be seen that the relatively large stripe width of prior art HBTs generally is a deliberate design choice and is not due to any processing limitation.